The present invention relates to document reproduction and more particularly to the removal of undesirable and unintended dots of toner or ink produced in a background area of a document. As will be detailed below, the primary use of the present invention is in connection with marking engines used in digital black/white and color copiers and printers. However, it is to be appreciated that the present invention can have uses in other fields where noise removed is beneficial.
FIG. 1 illustrates the image path of a color digital copier A, including scanner 10, image processing electronics 12 and printer 14. An original document is scanned by scanner 10, and converted into a stream of pixels. Image processing electronics 12 manipulates the pixel stream into a format suitable for use by printer 14, which selectively transfers toner or ink to a blank piece of copy paper, commonly a white piece of paper, thereby reproducing the images of the original document onto the copy paper.
Each scanned pixel is represented by three 8-bit values 20 that measure the intensity of the three primary colors (i.e. red, green and blue), such a representation being in what is known in the art as a 24-bit RGB color space. Black is represented by all three values of RGB being equal to 0, white is represented by all three values being equal to the maximum 8-bit value 255, and gray is represented by all three values being an equal value. In existing color digital copiers, the printer functions in a non-RGB color space which more closely mimics the human eye. Examples of such color spaces include CMYK, YCC, and LAB color spaces, the concepts of which are well known in the art. The whitest pixel produced by a CMYK printer consists of no toner or ink located at that pixel position. Therefore, the whitest pixel is limited by the whiteness of the copy paper.
FIG. 2 graphically represents the RGB color space where each pixel is represented by three 8-bit values (0-255). If a user wishes to increase or decrease the luminance of an output document, it is necessary to adjust each of the colors of the RGB color space by an equal amount such that the color of the pixels do not change, but rather only the luminance is altered.
On the other hand as can be seen in FIG. 3, which is a graphical representation of a color space such as YCC (where C0=the intensity value and C1 and C2 represent the hue of a color), intensity can be altered by changing the value of C0 without requiring a corresponding change to the C1 or C2 values. With attention to the C1 and C2 values of FIG. 3, the further away from the origin, the more saturated the color value (i.e. the deeper the hue). Also, if the C0 value is set to 0, then the values of. C1, C2 are irrelevant since the color will be black. Similarly, if the C0 value is made equal to its maximum, i.e. 255, then it does not matter what the values of C1, C2 are since the color will be pure-white.
Thus, by moving from the RGB color space to a color space more closely mimicking the human eye, such as the YCC or LAB color spaces, it is easier to control the luminance and hues of the documents being reproduced.
In order for scanners which scan images into the RGB color space and printers which operate in color spaces, such as CMYK, to function together in the same copier, a color space conversion takes place through the use of color space converter 16, whereby over 16 million colors in the RGB color space (2.sup.24) are converted to the 16 million colors of the YCC or LAB color space (2.sup.24). Thereafter, the pixels are provided to render 18 which converts the 16 million colors in the YCC color space to the 16 colors of the CMYK color space (2.sup.4) and then passes the pixels to printer 14. FIG. 1 also shows microcontroller 19, which provides intelligence to control operation of color digital copier A.
Another consideration regarding digital color copiers is that existing color marking engines (e.g. laser and ink jet) cannot satisfactorily reproduce extremely unsaturated colors (known as near-white). The resulting reproduction consists of a few widely scattered small dots of color (i.e. background noise), in the background of the copy paper. The term background refers to the region of the source document that has no image imposed on it so that the underlying paper is viewed. Common background types are white copy paper, colored copy paper, newspaper, magazine paper and photographs.
When the background of the source document is scanned, even if the paper appears white to a person, it appears off-white i.e. near-white, to a scanner. This off-white color is an extremely unsaturated color, hence the perceptually white background of the original is reproduced with colored dots scattered around the page. Viewed at a far distance, the region looks like the near-white original color but at a typical viewing distance the dots are quite noticeable and objectionable.
These near-white pixels are created by the combination of scanner inaccuracies, source document paper inconsistencies, and actual near-white pixels in the source document.
To overcome the above, the present invention provides a system that removes the undesirable background noise, in the form of unwanted dots of color, located in a background area of a document.